A friend dug this up in her yard and thought of me. It's a 1965 Multi Pistol cap gun from the Multi Pistol 09 spy set made by Topper Toys. It is fully functional, but has been very weathered from years of being outside and no longer has the shiny, chrome finish. I was able to remove a great deal of surfaced rust.

This is a video of the commercial for the gun where you can see how this small gun fits into the handle of the larger gun: